Blocked EGR pipe stopped low RPM miss fire

I bought a gen 3 Prius V with 400,000 KM (260k miles)
I went to clean out the egr due to p0401 code (egr flow insufficient) and high mileage.
I find a small square plate sandwiched in between egr pipe and egr cooler blocking the flow.
When I removed the blocking plate, engine misfires like crazy at low rpms.
putting back the blocking plate, the engine runs fine but give p0401 code. what gives?
current egr cooler is likely clogged but how come misfires stop when a blocking plate is put in?
could this be the new easy way to fix low RPM misfires?
im thinking manually blocking the egr is not good for head gasket long term.

UPDATE: I figure the EGR is stuck open so that explains the misfires at low RPM without blocking plate.